1. Field of the Invention
The invention relates to a transflective display device, and more particularly to a transflective display device that employs a photo-alignment process to make liquid crystal molecules have at least two pretilt angles within a single cell gap.
2. Description of the Related Art
Liquid crystal display (LCD) devices are usually classified into transmission type and reflection type according to the difference in their display light source. The transmission type LCD device uses a back light module, in which the light is incident to an LC layer and is absorbed or passes through the LC layer, thus disadvantages of faded color and reduced contrast ratio are found under a natural light source or an exterior artificial light source. On the contrary, the reflection type LCD device uses an ambient light incident from an exterior light source, resulting in superior performance and high contrast ratio under outdoor sunlight. Also, because of its low power consumption, the reflection type LCD device is focused on portable display products. However, the reflection type LCD device is useless when the weather or exterior light source is dark, and it is comparatively difficult to achieve high resolution for a full color display.
Accordingly, transflective LCD devices have developed to compensate for the reflection type LCD device and possess the advantages found in the transmission type LCD device and the reflection type LCD device. The transflective LCD device can use well known active driving processes, such as amorphous silicon thin film transistor (a-Si TFT) or low temperature polysilicon (LTPS) TFT, and is applied to information products of low power consumption. U.S. Patent Application Publication No. 2002/0003596A1 discloses a transflective LCD device that designs an LC cell as dual cell gaps and has a retardation film.
FIG. 1A is an exploded perspective view illustrating a typical transflective LCD device. FIG. 1B is a cross-sectional view illustrating dual cell gaps of the transflective LCD device shown in FIG. 1A.
In FIG. 1A, a transflective LCD device 1 includes an upper substrate 10 and a lower substrate 20 opposing each other, and an LC layer 30 is interposed therebetween. For the upper substrate 10, on an inner surface opposing the lower substrate 20, a black matrix 12 and a color filter layer 14 that includes a plurality of red (R), green (G), and blue (B) color elements are formed, and a common electrode 16 is formed to cover the color filter layer 14 and the black matrix 12.
For the lower substrate 20, on an inner surface opposing the upper substrate 10, a plurality of crossing gate lines 26 and data lines 28 are formed to define a plurality of pixel regions P arranged in a matrix and corresponding to the color filter layer 14, and a plurality of TFTs, serving as switching devices, is located near each cross point of the adjacent gate line 26 and data line 28. Also, a plurality of reflective electrodes 22 and a plurality of transparent electrodes 24 are formed on the pixel areas P, respectively. Each of the reflective electrodes 22 has a through hole to expose a transparent electrode 24 disposed there below. Thus, the exposed portion of the transparent electrode 24 serves as a transmissive region T, and the electrode portion outside the through hole serves as a reflective region R.
In FIG. 1B, the lower substrate 20 comprises a protection layer 36 sandwiched between the reflective electrode 22 and the transparent electrode 24, and the through hole passes through the reflective layer 22 and the protection layer 36 to define the transmissive region T and the reflective region R. Furthermore, an upper alignment layer 32I is formed on the common electrode 16, a lower alignment layer 32II is formed on the reflective electrode 22 and exposed portion of the transparent electrode 24. A half wave plate (HWP) 34 and an upper polarizer 38I are sequentially disposed on the exterior surface of the upper substrate 10, and a lower polarizer 38II and a back light 40 are disposed on the exterior surface of the lower substrate 20. The HWP 34 is employed to involve a phase difference of λ/2 for incident lights.
Accordingly, in the pixel area P, the LC layer 30 has a first cell gap d1 over the reflective electrode 22 within the reflective region R, and a second cell gap d2 over the transparent electrode 24 within the transmissive region R. The second cell gap d2 is twice beneficial as the first cell gap d1. Thus, a light achieves a phase difference of λ/4 after passing through the first cell gap d1, and a light achieves a phase difference of λ/2 after passing through the second cell gap d2. After passing through the HWP 34, the light achieves the additional phase difference of λ/2. The phase retardation in the transmissive region T is twice the phase retardation in the reflective region R.
Nevertheless, for patterning dual cell gaps in the LC layer 30, the process becomes more complicated. Also, the transflective film, such as the HWP 34, adhered to the exterior surface of the upper substrate 10 cannot achieve superior performance. Accordingly, a transflective LCD device based on a single cell gap design to achieve optical demands for both the transflective region and the reflective region is called for.